The use of enhanced boiling surfaces, and in particular nucleate boiling surfaces to increase the heat transfer film coefficient on boiling side heat transfer surfaces is well-known. U.S. Pat. No. 4,769,511 discloses the use of an enhanced nucleate boiling surface to improve the operation of a process for the alkylation of isoparaffins. The use of nucleate boiling surfaces to improve the operation of heat exchange equipment in the reforming of hydrocarbons is disclosed in U.S. Pat. No. 5,091,075.
Although enhanced boiling surfaces will usually improve the operation of exchangers and the processes in which the indirect heat exchangers operate, it has been unexpectedly found that enhanced boiling surfaces in some boiling applications will provide little or no benefit. The achievement of little or no benefit from the addition of an enhanced boiling surface was difficult to understand. Enhanced nucleate boiling surfaces greatly improve the heat transfer coefficient across the boiling film. An enhanced boiling surface having a porous boiling layer should provide about a ten fold increase in the nucleate boiling film coefficient. The contribution of the enhanced nucleate boiling surface in providing an improvement in overall heat transfer when used in flow boiling on the inside of a tube, of course, depends on the contribution to the heat transfer associated with nucleate boiling across the film and the contribution associated with convection. In some flow boiling applications, as much as 85% of the contribution to the overall heat transfer is attributable to convection. Nevertheless, in such cases even a 15 to 25% contribution of the nucleate boiling film heat transfer coefficient will allow the use of an enhanced boiling surface to demonstrate significant improvements in the overall heat transfer coefficient. Thus as long as their is some vapor generation, the addition of such a surface should provide a significant increase in the overall heat transfer performance. Moreover, in several instances data showed that nucleate boiling surfaces achieved less than a quarter of the tube side boiling heat transfer coefficient predicted by calculation.
A specific case where an enhanced boiling surface in a heat exchanger provided no benefit was in sulfuric acid alkylation process. The basic reaction of this process is the conversion of isobutane and isobutene in the presence of concentrated sulfuric acid to produce iso-octane. This process is well described in U.S. Pat. No. 4,769,511, the contents of which are herein incorporated by reference. In this process, olefins and isoparaffins are mixed on the shell side of a contactor that contains a plurality of tubes for indirect heat exchange. The product of the reaction is an emulsion of sulfuric acid and alkylate products that is decanted in an acid settler to separate a hydrocarbon stream containing the alkylate products from the acid. In a typical operation, the separated hydrocarbon stream undergoes a Joule-Thomson expansion that cools the liquid and generates a substantial amount of vapor. This two phase mixture of hydrocarbon liquid and vapor enters the inside of the heat exchange tubes in the contactor where further vaporization of the hydrocarbon stream removes heat generated by the alkylation reaction taking place on the shell side of the contactor.
It has been surprisingly found that heat transfer tubes coated with an enhanced porous boiling surface on their inside perform at essentially the same heat transfer rates as bare tubes. It was also unexpectedly encountered that tubes having an interior coating with an enhanced boiling surface demonstrated the same dependence on tube side flow rate as tubes with a bare interior wall. Since an enhanced boiling surface will provide a nucleate boiling heat transfer coefficient for the interior of the tube that is approximately ten times greater than that for a bare tube, the failure to observe any increased performance from the addition of the enhanced boiling surface was difficult to understand.
Moreover, it was unexpectedly found that the bare tubes provided heat transfer rates much below those predicted by known heat transfer correlations. This lower than expected performance was in comparison with correlations that account for stratified flow. Thus, the cause of the observed low heat transfer performance has been difficult to understand.
It is an object of this invention to improve the performance of boiling surfaces in mixed phase flow applications.
It is a further object of this invention to provide a method of achieving higher overall heat transfer rates for an enhanced boiling surface relative to a bare tube surface when contacting the tube surfaces with a mixed phase flow.
Another object of this invention is to improve the operation of acid contactors in alkylation processes that use boiling surfaces in multiple tube arrangements.
A further object of this invention is to provide a heat exchanger that advantageously uses an enhanced boiling surface to vaporize a mixed phase stream by indirect heat exchange.